Telegraph exchange system



May l2, 1931. G. s. VERNAM 4 TELEGRAPH EXCHANGE SYSTEM Filed April .'5. 1929 4 Sheets-Sheet l llwwl/l Q S@ Q A 6'; S. @Firman/U BY mii-:Y

May 12, l931. G. s. VERNAM 1,804,548

TELEGRAPH EXCHANGE SYSTEM Filed April 3. 1929 4 Sheets-Shet 2 TTORNEY May 12, 1931. G. s. VERNAM 1,804,548

TELEGRAPH EXCHANGE SYSTEM Filed April 5. 1929 4 Sheets-Sheet 3 Conmecz'l/y C0765 Repeater Cord BYl 241% ATTORN EY May 12, 1931. G. s. VERNAM TELEGRAPH EXCHANGE SYSTEM Filed April .'5. 1929 4 Sheets-Sheet 4 NNN lNvENToR 5. n/alam Patented May l2, 193i UNITED STATES PATENT OFT-"ics GILBERT S. V'EBNAI, OF EDGE, NEW JERSEY, ASSIGNOB TO AMERICAN TELE- PHONE ANI) TELEGBAIH COMPANY, CORPORATION 0F NEW YOBK l'.l'IEIIIGRAIPH EXCHANGE SYSTEM v Application ld April 3,

This invention relates to switching systems, and more particularly to such systems as applied to telegraph circuits, and discloses an exchange system applicable principally to the use of printing telegraph equipment,4 by means of which connections may be established quickly and eiliciently for communication purposes between any of a plurality of subscribers telegraph stations, and further by means of which such connections are to remain thus established for more or'less temporary intervals in accordance with the wishes of the subscribers.

In short, the invention aims to provide between subscribers telegraph stations the same type` of service that is now provided between telephone stations. At the present time, any telephone user may obtain a connection with any other such user by merely signaling the operator. In a similar manner, the telegraph exchange system disclosed herein provides means whereby any telegraph station served by the exchange system may obtainconnection with any other such station by merely signaling the operator at a central oiiice to which the telegraph station is connected.

Accordingly, a primary object of the present invention is to disclose a exible switching system for quickly establishing and tearing down communication circuits `between any of'a plurality of subscribers printin telegraph stations, or between a subscribe s station and an operators printer set located at a central oiiice.

Another objectoffthe invention is. to disclose suitable signaling means associated with` such a switching system whereby a subscriber may signal an operator, or `vice versa, or whereby an operator may signal a distant operator.

Other objects of the invention will appear4 in the subsequent detailed disclosure.

The telegraph exchange system, broadly speaking, comprises a group of subscribers telegraph stations each connected to a given centraloiiice over a loop circuit, withother groups of subscribers stat-ions similarly connected to other central oilices.. Telegraph trunk circuits extend between the. central oilicesV in question, and link circults are pro- 1929. Serial No. 852,186.

need due to the limitations of the present types of telegraph service available to the telegraph user. At. the present time, the telegraph user must either transmit his messages through the usual public telegraph oillice or else must lease a private wire to some distant point for a considerable period of time. The former type of service is, of course, intended for the occasional message, while the latter type of service is intended for the user who has a large amount ofdaily business with a single distant station.

Since a telegraph user is in general located at some distance `from the public telegraph oflice, delay and possible error is introduced by handling messages in this way. The leased Wire, while expeditious, is expensive and provides communication only with a single distant station. N ow, it is felt that `there area large number of telegraph users who have not enough business with a given point to render a private wire thereto economical, alud who at the same time have business of such a character that the messages must 'be transmitted expeditiously.

To a. telegraphuser of this type, the telegraph exchange system disclosed herein would furnisha solution of the problem. By having his office equipped with a printing telegraph sending and receiving set, such a Auser could be connected directly to a desired distant station for any desired period of time during which he had business to transact. During this period, the user would have in eifect a private line to the distant point. His business could, therefore, be transacted expeditiously and yet he need pay for the telegraph service only during the time he actually used the same. During other intervals, the trunk circuits extending between oilices other stations.

Having thus broadly outlined the purpose and scope of the present invention, the detailed operation of the same will now be considered with reference to the drawings, of which:

Figure 1 shows in schematic form the essential elements of va complete telepgraph exchange system embodying the novel Afeatures of the presentinvention.` Fig. 2 shows the circuit diagram of a subscribers loop circuit includin the apparatuaassociated with the loo at t e central oiice as well as at the subscribersstation. Figs. 3 and 4 show link circuits or connecting cords which are used in establishing connections between subscribersZ lines, between trunk circuits or between a subscribers line anda trunk circuit. Fig. 5

lGeneral features of ig/.stemtv The telegraph exchange system shown in Fig. 1 comprises three central oices A, B and C. Telegraph trunk circuits 1 and 2 connect oices A and B, while trunk circuits 3 and 4 'connect oiices B and C. Olce B is assumed to be situated at an intermediate point between oilices A and C, and hence no vtrunk circuits extend ldirectly from oilice A to .ofiice C. A plurality of subscribers stations 5, 6 and 7 are connected over individual loop l, circuits to oiice A. Similarly, subscribers stations 8 and 9 are connected to oiiice B, and stations 10 and 11 to oiiice C'. Each central o iice is equipped with an operatrs set 13 similar` to that shown in Fi 5, and a disconnect slgnal circuit 14 simir to that shown v1n Flg. 7. Each central oilice is equipped with a plurality of connectin cords 15 and 16 forestablishing connections etween the-subscribers. stations, subscribers stationsV and trunk clrcuits, or between trunk circuits. The repeater cords 16 are used for connecting subscribers lines which terminate in the same oiiice, or for interconnecting trunks on builtup connections while connecting cords 15 are used for connecting a subscribers line to a trunk circuit. A l

Each subscribers loop circuit and each 50 trunk circuit terminatesat the central oflice in a group of three jacks marked R, L and C -911 the drawing. J ack R is the repeater )ack 'and is intended to receive a plu from a -:repeater cord 16, while jack C is t e connccting jack and is intended to receive a' between the repeater jacksof the respective loop circuits. A trunked circuit connection between distant subscribers is indicated as established between stations 11 and 8. At

`oiiice C, connecting cord 22 extends between the connecting jack for station 11 and the connecting jack of trunk 4,-while atoiiice B connecting cord 23 extends from the connecting jack of the trunk to the connecting jack of station 8. A built-up connectioni between distant stations is shown established between stations 5 and 10. This circuit is completed from station 5, over connecting` cord 18, trunk circuit 2, repeater cord 19 at oiiice B, trunk f circuit 3 and connecting cord 20 to station 10.

With the exchange system to be described it is possible to establish so-called conference connections, i. e., a connection involving more than two subscribers stations. A connection of this type is indicated foi' stations 5, 9 and 10, station 9 being associated with the circuit connecting stations 5 and 1G by means of the connecting cord 21 plugged into the connecting jack of trunk 3 at oflice B. These conference connections may be established between subscribers lines terminating in the same oice as well as on trunked circuit connections.

g Local connection v The ex lanation given above in connection with ig. 1 furnishes a general picture of the exchange system operation as a whole. The individual types of circuit connections will now be taken up in detail by first considering a local connection, i. e., a connection between two subscribers having loop circuits terminating in the same central oice. A connection ofthe type here contemplated is similar to that shown in Fig. 1 between stations 6 and 7.

Referring to Fig. 2 which shows the subscribers loop circuit, the equipment .to the left of the line :zi-ai is located at the central 'oiice, while theequipment to thevright of line y-y is located at the subscribers station. The equipment aty the subscribers station Vcomprises a printing telegraph sending unit 30 and a receiving unit 31, a break key 37 a switch 33 for startingk the printer set motor 32 and also forsignaling the central' 13 4switch 36 the functions of which will be explained later. v

At the central station the loop circuit terminates in the jacks R1, L1 and C1, the subscripts 1 being utilized to distinguish the loop circuits from the trunk circuits discussed later, the jacks of the latter circuits being designated by subscripts 2. A li'ne lamp 39 is associated with the acks whereby the subscriber may signal the central station, and there is an auxiliary signal circuit 40 controlled by the circuit of lamp 39. In addition, there are provided severall relays, to be discussed, for operating the signaling circuits.

The repeater cord` which is necessary for establishing a connection between local subscribers lines is shown in Fig. 4. This cord contains a two-way telegraph repeater circuit for repeating signals in either direction between lines toivhich it is connected by means of plugs 69 and 70. The repeater unit comprises a group of four polarized relays to 6.8, inclusive, each havinor line and auxiliary windings interconnected as shown. The auxiliary windings aresupplied with current from the power source 71, as indicated. A detailed explanation of the operation of this repeater station will not be given in this application, since a repeater of this type is fully described in the U. S. Patent No. 1,479,430, to J. M. Fell, dated J anuary 1, 1924, which patent is hereby made a part ofA the present application as if fully included herein.

Fig. 5 shows the printer circuit of the operators set which is used in communicating with subscribers in establishing connections. .The operators set terminates in a plug and cord 7 5 so that it may be connected to the jack of a subscribers line for communication purposes. In series between the` tip land sleeve of the plug are the sending contacts 76 of the printer sending unit, 4the break key 82, and the polar relay 77 which operates the receiving unit of the printer, to-

` gether with a key which is used for ringing on the subscribers line, as explained below. The elementl 78 is the printer selectlng magnet which controls the operation of 'the printer receiving unit including the select ion of-characters, this printer magnet 78 being in turn controlled by the line relay 77, as explained below.

Having briefiy discussed above the varlous f circuit elements involved in establishing a connection between local subscribers lines, the manner in which this connection is established will now be taken up. Referring again to Fig. 2, assumethat the subscriber desires to initiate a call. This is accomplished by operating key 33, which connects the source of power 41 to the printer motor 32 and thus startsthe motor. In addition, the operation of key 33y completes a circuit to light line lamp 39 vat the central oiice. This circuit is traced from battery 50 at the central oce,

through the non-operated contacts of the.

jacks C1 to L1, in series, respectively, through the upper winding of relay 53 and over the S conductor of the loop circuit 60 to the subscribers station, and thence through the lower operated contacts of key 33, through left and right non-operated contacts of switch 36 through the printing telegraph receiving and sending units 31 and 30, respectively, and break key 37 of printer 57 through left nonoperated contacts of key 36, back over the T conductor of loop 60, to the central station, through contacts of jacks M and N and over lead 63 thereat, through the lower back contact and armature of relay 54, upper nonoperated contact of jack C1, upper armature and back contact of relay 54l to ground through the left armature and back contact of relay 52. Current flowing in this circuit operates relay 53 at the central oiice, which completes a circuit through its front contact and armature to operate relay 52. The operation of relay 52 completes a circuit from battery 64, through the right inner front contact and armature of relay 52, through lamp 39 and relay 45 to ground. Lamp 39 lights over this circuit, and relay 45 voperates and completes the auxiliary signal circuit 40 to light pilot lamp 42.

The pilot lamp 42 lights every time a call is initiated on any loop circuit connected to the switchboard, due to the factthat the operating circuits for the lamp 39 individual to all the loop circuits are multipled to relay 45 as indicated by lead 49. Thus, the pilot lamp 42 indicates to the operator every time a callL is initiated, While the individual lamp 39 shows the particular loop circuit initiating the call. If desired, the operation o/f relay 45 may be made to operate a buzzer circuit by merely operating key 44 to connect relay 46 in series with the lamp 42. With this arrangement, when relay 45 operates, lamp 42 will light, and also relay 46 will operate to close the circuit containing batteryv 48 in series with buzzer`47. The buzzer will thus operate and serve as a warning that a call is waiting on one of the loop circuits.

The operator at the central oiice answers the call by inserting plug 75 of her set, Fig. 5, in jack L1 of the subscribers line.- This extinguishes the lamp 39 and connects the operators printerv set to the loop circuit for communication with the subscribers printer.

Upon insertion of plug 75 in jack L1 a circuit the sleeve conductor of lug 75 and jack L1, through the upper win yin of relay 53 and out over the S conductor -o the loop circuit, through therv subscribers printer set in the manner traced above, and back over the T loop conductor to the-central oiice, throu h contacts of jacks M and N, and through t e lower winding of relay 53, and through the lower front contact and armature and upper armature and front contact of relay 54 to positive battery 51. This current flowing through both windings of relay 53 causes the release of the relay which, it. will be noted, 1s differentially wound. Upon the release of relay 53 relay 52 releases, in turn disconnecting battery 64 from lamp 39, which extinguishes the lamp. v

The operator now operates the printer set of Flg. 5 to send the word operator or opr, which is typed up on the receiving unit 31 of the subscribers printer. The subscriber then answers by operating the sender unit 30 to transmit to the operators printer set information regarding the called subscrlbers line. The operator, having obtained this information, connects plugV 'fof a repeater cord (Fig. 4) in the repeater jackvR1 of the calling subscribers line and connects plug 69 thereof in'jack R1 of the called subscninns line. :Both the calling and @auna subscribers, of course, have loop circuits similar to Fig. 2. The'insertion of plug 69 in jack R1 of the called subscribers line operates relay 54 tov ground through the operated contacts of the jack. The o eration of rela 54 completes a circuit to liglht lamp 39 of t e called subscribers line. This circuit 1s traced from positive battery 51 through operated contacts of relay 54 and non-operated .contacts of jack C1, through the lower wmdmg of relay 53 and out over the T conductor of the loop to the subscribers station, through left non-operated contacts of key swltch 36 thereat, through the printer unit 57, through right and left non-operated contacts of key switch 36 to ground 61 through lower non-operated contacts of switch 33 Relay 53 operates `over this circuit and completes the circuit to operate relay 52, as explained above, and the operation of relay 52 1n turn llghts lamp 39.

The operator now removes plug, from ]ack L1 of the calling subscribers line and 'inserts it in jack L1 of the called subscribers h ne and rings on the circuit by operating rlnglng key of Fig.l 5. The operation of key 80 completes a circuit from the ringing generator 79, through the right operated contacts of switch 80, through the sleeve contacts of plug 75 and jack L1 of the called subscribers line, through the upper winding of relay 53 and out over the S conductor of the loop to the called subscribersstation and through the ringer 34, condenser 35 and nonloperated contacts of switch 36 thereat to ground 61 thrnngh switch 33. The ringing current thus rings the bell 34 at the called y subscribers station.

The called subscriber answers by operating l switch 33 associated with the printer. set

through the lower winding of relay 53 and through operated contacts of relay 54 to positive battery 51. The current in both w1nd ings of relay 53 releases the same, as explained, which in turn causes lamp 39 to be extinguished.

The called subscriber now acknowledges the call in some way, as by sending his number on the printerhwhich is received on the printer sets of both the operator and the calling subscriber. The operator monitors on the connection until the parties are in satisfactory communication, and then disconnectsA her set by removing plug 7 5 from jack L1 of the called subscribers line.

When the call is com leted both subscribers restore switches 33 to t eir normal positions, as shown in Fig. 2, which stops the rinter set motors thereat and also opens t e circuit f the S loop conductor at the contacts of switch 33, which prevents the further iiow of direct current in the upper winding of L1, through the u per.

relay 53. The current flowing through the f lower windingthereof thereupon re-operates the relay which, in turn, causes the lighting of lamp 39 associated with the loop. The operator, upon noting the lighted lamps 39, disconnects the repeater cord from the R1 jacks of the calling and called'subscribers lines, thus' restoring all equipment to norma Tru/nk circuit connection The establishment of a trunked circuit connection is` somewhat different from that outlined above for a local connection. Referrin plugs connected tip to tip and sleeve to sleeve.

The circuit diagram of one termination of a trunk circuit is shown in Fig. 6. Tt is only necessary to show the termination of the -trunk circuit at one central ollice, since it is identically terminated at each office between which it extends. Referring to the drawing. the trunk terminates at each oirice in three jacks Rg. L2 and C2. These jacks correspond to the repeater, listening and connecting cord jacks, respectively. As was mentioned above, the subscript 2 is used in this case merely to distinguish the jacks from the corresponding jacks of a subscribers line (Fig. 2) which have the subscript 1. `The equipment shown to the right of the line .af-z in Fig. 6 is novel to the present invention, whereas that to the left of the line z-z comprises standard telegraph equipment which is well known to the art. The trunkcircuit shown utilizes alternating carrier current for transmitting the telegraph signals over the trunk circuit, although any othery method of signal transmission would be quite satisfactory for the present purpose.

The equipment to the left of the line 2 2 A serves to convertl the D. C. telegraph pulses into A. C. carrier current'vpulses for transmission over the telegraph line 104 to the distant end thereof,andalso serves to convertathe A. C. carrier pulses received from the distant end of the line into D. C. telegraph pulses. Thus, with the printer set connected'to jack L2, R2 or C2 the operation of the sender unit will cause corresponding carrier current pulses to be transmitted over line 104. 0n the other hand, carrier current pulses received over the line will be converted into D. C. pulses Vsuitable for operating the receiving unit of the rinter set.

No detailed explanatlon will be given herein as to the method of operation of the carrier translating equipment as a circuit of this general type is fully discussed and explained in U. S. lPatent No. 1,469,259, to B. l?. Hamilton, dated October 2, 1923. Speciical-- 'ly, Fig. 1 of the Hamilton patent 'discloses a circuit arrangement almost identical with that shown to the left of line z-z (Fig. 6).

To explain briefly, however, how the circuit operates, assume plug 7 530i the operators printer set 5 to be inserted in jack L,

of Fig. 6. lhis will complete a circuit from positive battery 108, through resistance 96, equivalent to the resistance of a subscribers loo circuit, through non-operated contacts of Jack C2, through the operatorls printer set connected to jack L2,.and back throughnonoperated contacts of jackl R2, through relay 927 resistance 113 and the line windings of re-V lays 99f and 97, the upper arm of impedance bridge 114, and the armature and marking contact of polar relay 107 to ground through negative battery 109. Current owing in this circuit operates relays 99 and 97 against their marking contacts, as indicated. A second current ows from negative battery 109 through contacts of relay 107, through the lower arm of impedance bridge 114, and the current in the line windings of relay 97 and' 99 will permit the current in the auxiliary windings to move the armatures thereof against their spacing contacts, and thus the armatures of these relays will follow the signals transmitted from the operators printer set.

Relay 99 is the sending relay which, by alternately closing and opening the short-circuiting path across theoutput of oscillator 101, controls the transmission of current from the oscillator through band-pass filter 102, through hybrid coil 103, and out over line 104, in accordance with signals transmitted from the oprators printer set.

Carrier current signaling pulses received over line 104 from the distant point flow through hybrid coil 103 into receiving circuit .111, and thence through band-pass filter 105 into detector 106. The rectified current in the detector circuit flows through relay 107 and operates the same in accordance with the signals received. Relay 107 has normally negative battery 109 connected to the marking contact, and positive battery 112 supplied to the marking contact and armature of relay 97 connected to the spacing contact. As relay 107 operates, therefore, in accordance with the received signaling currents, it connects in series with the positive battery 108 through the operators printer set, alternately negative battery 109- and positive battery 112. Thus, while rela 107 rests on its marking contact current ows through the operators printer set, and while it rests on its spacing Contact no current flows therethrough since positive battery 112, which is equal in magnitude to positive battery 108, opposes the same through the operators printer setand thus prevents the ow of current therethrough.

The line relay 77 of the operators printer set will, therefore, follow the received signaling pulses and cause the received message to be typed up due to the resulting operation of the selecting magnet 78.

Relays 97 and 99 are not operated by the l received signaling pulses since with relay 107 on its marking contact, the How of current through the line windings of these relays operates thevsame against their marking contacts, whereas with relay 107 on its spacing contact, nocurrent Hows in the line windings of relays 97 and 99 and the reversed current when t naling pulses. Under these conditions, relay 97 operates in such manner as to maintain re- Yk-lay 99 against its spacing contact during transmission of the break signal, and thus permits the local operator to transmit a clean break signal to the distant point. The manner in which this is accomplished is fully set forth in the patent to Hamilton mentioned` above, to which reference is made at this point for a more complete-description of the end of the trunk.

circuit operation.

Returning now to the establishment of a trunked circuit connection between distant subscribers stations, the call is initiated in the` manner described above for a local connection. Referring to Fig. 2, thevcalling subscriber operates switch 33 to start the printer set motor at the calling station and to signal the central oice. The operator at the central oiiice responds by connecting her set to the L1 jack of the calling subscribers line and obtains the necessary information in regard to the called station.

When the operator has learned that the call is intended for a station connected to a distant oiiice, she inserts one end of a connecting cord (Fig. 3) in the C1 jack of the calling subscribers line, and inserts the other end thereof in the C2 jack of an idle trunk (Fig. 6) terminating at the desired otlice. The operator then signals the distant office by operating the break key 82 for about onehalf to one second which causes the line lamp 90 at the distant end of the trunk to light.

The operation of the break key interrupts current iiowing in the circuit traced from positive battery trunk, jack C2 and sleeve conductor of the connecting cord, jack C1, calling subscribers loop circuit, sleeve conductor of jack L1, operators set (Fig. 5), tip conductor of jack L1, non-operated contacts of jack R1, )ack C1 and tip conductor of connecting cord, jacks C2, L2 and R2, winding of relay 92 and through the sending relay 99 tol ground at negative battery 109. Relay 99 is thus operated against its spacing contact causing transmission of a break signal to the distant Since no plug is inserted in j ack R2, L2 or i' C2' at the distant end of the trunk, relay92 thereat is normally operated iover the circuit traced from positive battery 108, through resistance 96, and in series through non-operated contacts of the jacks, the winding of relay 92 and thence through the intermediate equipment to ground through negative battery 109. When, however, the break sigtive battery 109, and the positive battery; `winding of relay 92 and 108 in the local end of the nal transmitted from the callin end of the trunk arrives at the distant en it operates relay 107 thereat against its spacing contact. This substitutes positive battery l112 for nega- 112 the opposes the positive battery 108 throu lease of the relay.

Now, relay` 92 controls the circuit for o erating relay 91, which in turn controls t e lighting of lamp 90. Associated with relay 92 are the condenser 93 and resistance elements 94 and 95. While relay 92 is operated no charge can accumulate on condenser v93 since both terminals thereof are connected to battery 98, one terminal being connected directly thereto through resistance 100,

,and the other terminal being connected thereto through the winding of relay 91 and the upper front contact and armature of relay 92. When, however, relay 92 releases, as explained above, due to the reception of the break signal, the upper terminal of condenser 93 will be disconnected from battery 98 at the upper contacts of relay 92 and in place thereof will be connected to ground throu h the armature and lower back contact of .t e relay and through resistance elements 94 and 95 in parallel. As a result, whilerelay 92 is released, the condenser 93 will slowly charge up to the potential of battery 98, so that if the breaksignal is transmitted long enough the condenser` will be fully charge/d tothe battery potential when relay 92 is reoperated at the termination of the break signal. The combined resistance of elements `94 and 95 in parallel is such that forthe us causes t e recircuit indicated the condenser `93 will be y' fully charged in about one-half to one second.

After the operator at the calling oilice has held break key 82 thereat operated for the desired length of time she' releases the same, causing relay 99 thereat to move against its marking contact and transmit a marking signal to the distant end of the trunk. The marking signal, upon arriving thereat, operates relay 107-against its marking contact, thus causing a reoperation of relay 92. The operation o relay 92 reconnects the winding of relay 91 across the terminals of condenser 93 so that the condenser discharges through the relay winding and operates the samemomentarily. As soon as relay 91 thus operates it completes a locking circuit for itself. This circuit is traced from positive battery' 98 through the windin lower armature and front contact of re ay 91 to ground 112 through non-operated contacts of jack L2. Relay 91, upon operating, connects battery 115 to lamp 90, thus lighting the lamp at the distant end of the trunk. Lamp 90 may, ify desired, be wired through an auxiliary signaling circuit similar to that shown at 40 of Relay 92 at the calling end of the trunk will be likewise released during transmission of the break signal, but will not cause the operation of 'relay 91 to light lamp 90 thereat upon termination of the break signal. yThe explanation of this difference in operation at thecalling and called ends of the trunk is to be found in the different rate at which condensers 93 are charged at the two ends.

llt will be noted that resistance 95 normally obtains ground through non-operated contacts of jacks C2 and R2 inv series. With no plug inserted in jacks `R2 or C2 the condenser 93 is charged through resistances i 94 and 95 in parallel, in which case the condenser will be charged to the full potential of battery 98 in one-half to one second. 1f, however, a plug is inserted in jack R2 or C2 the ound is disconnected from resistance 95, so t at condenser 93 must charge through resistance 94 only. Now, resistance 94l is very much larger than resistance95, and in fact, is so proportioned with respect to the capacity of condenser 93 as to require sixy to eight seconds for charging the same to the full potential of battery 98. I

Returning now to the circuit connection under consideration, it will be recalled that the plug of a connecting cord is inserted in the C2 jack at the calling end of the trunk, while no plugs are inserted in any of the `jacks at the called end. rllhus the transmission of a break signal of one-half to one second duration in the manner explained above will permit a full charge to accumulate on condenser 93 at the called end of the trunk, but only a small charge to accumulate on the corresponding condenser at the calling end. As a result upon termination of the break signal,

relay 91 will be operated at the called end to light lamp thereat, but will not operate at the calling end due to the small current discharge therethrough from condenser 93.

Incidentally, at this point it is well to bring out the fact that the insertion of a plug in al1- L2 jack of the trunk will prevent more than a momentary lighting of lamp 90 at the corresponding end of the trunk, due to the fact that the locking circuit for relay 91 obtains ground at 112 through non-operated contacts of'jack L2. The insertion of a plug 1n this jack of course disconnects the ground and prevents the relay from locking up.

lln a specific designthe following constants were used for the elements 93, 94 and'95. Capacity 93 was taken as 2 mf., resistance 94 as 2 megohms and resistance 95 as 100,000 ohms.

Returning now to the establishment of the -connection with the called subscriber over the trunk circuit, the operator at the distant end ot the trunk, upon noting the lighted lamp 90, inserts plug 7 5 of lier .operators set, similar to Fig. 5, in jack L2 of the trunk, which removes the ground 112 from relay 91 thereat and extinguishes lamp 90. The distant voperator then .w1 u with the calling operator and obtains the number of the called line. lf thecalled line is idle the distant operator inserts a plug of a connecting cord (Fig. 3) in the C2 jack of the trunk circuit and inserts the other end of the cord in the C1 jack of the called subscribers line Y and operates the ringing key 80 of her operators set which, it will be recalled, is still connected in the L2 jack at the distant end of the trunk. The ringing current in this case flows from the generator 79 (Fig. 5) through the right operated contacts of key 80, through the sleeve circuit of plug-7 5 and jack L2 (Fi 6) and out over the tip conductor of jack g2 (Fig. 6) and through the tip conductor of the connecting cord (Fig. 3) to the tip terminal of jack Cl (Fig. 2) through non-operated J contacts of jacks R1 and L1, the upper winding of relay 53 of the called subscribers line, out over the S conductor of the loop and through lringer 34 to ground 61 at the called subscribers line. The ringer 34 thus oper- .ates to signal the called subscriber that a call is waiting on the line. 1t will be noted that while key 80 (Fig. 5)-is operated, positive battery 83 is connected through the left operated contacts of key 80 to the tip of plug 75. This battery 83 replaces the positive battery 108 of the trunk circuit during the ringing operation and hence prevents the transmission of a break signal back over the trunk circuit duringr the ringing interval. Owing to the make-beore-break arrangement of the left contacts of key 80, battery 83 is connected to the trunk circuit before battery 108 is disconnected, thus avoiding any possibility of current interruption during operation' of the key over the T loop conductor to ground 61 at the called subscribers station. The operation of relay 53 in turn -operates relay 52 as explained which lights lamp 39 associated with the called line.

1n addition, the operation of relay 52 at the time completes a circuit from positive battery 51, right outer front contacts of rela;v 52, resistance 59,'non-operated contacts of jacks L1 and R1, tip conductors of jacks C1, the

connecting cord and-jack C2 of the trunkV clrcuit, the operators set connected to jack L2, non-operated contact of jack R2, and windings of relays 92, 99 and 97 to ground through negative battery 109. Current dowing in this circuit maintains relay 99 at the 1Upon establishing connection` with the 'l positive battery 108 in called end of the trunk operated against its marking contact, and hence prevents transmission of a break signal back over the'trunk circuit at this time. Were it not for this feature, a break signal would be transmitted to the calling end of the trunk which would cause a charge to accumulate on condenser 93 thereat as explained and might possibly cause the` operation of line lamp 90 thereat upon the subsequent operation of switch 33 at the called station to prevent further transmission of the break signal as explained below. Resistance 59 is given avalue about equal lto that of the subscribers loop resistance.

When the called subscriber answers, the operation of switch 33 at the called station connects the loop circuit between batteries 109 and 108 of the trunk circuit, which causes the same current to ow through both windings of the diierential relay 53. Relay 53 thereupon releases, in turn releasing relay 52, whlch extinguishes lamp 39 and opens the c1rcu1t traced above for supplying current to the trunk circuit from battery 51 throughresistance 59.

'.Ilhe called subscriber now operates his prlnter to aclmowledge receipt of the call, which acknowledgment is typed up on the printers of the operatorssets associated with both ends of the trunk and also on the calling subscribers printer. The operators monitor on the connection until the calling and called arties are in satisfactory communication, w ere'upon they disconnecttheir sets.

During communication the subscribers rmay break each other in the usual way, but these break signals will in general not operate the lamps 90 associated with the trunks, since as explained, with connecting cords in the C2 trunk jacks, a break signal of say eight to ten seconds duration is required to lnsure lcomplete charging of the condensers 93, and

the average length of a subscribers break signal is in general of considerably less duratlonthan eight to ten seconds. If, perchance, the hne lamps 90 are operated subsequent to an exceedingly long reak signal, it will merely result in the operatorsgcoming in and monitoring on the connectiolilfor a *short interval. i

While the subscribers are thus connected othe trunk, ani-operator may, if desired,

rm 'brea signal of eight to ten seconds to be transmitted over the trunk which will Jight the-'distant line lamp 90as` explained. This longA break signal could be transmitted by associating lthe o jack and holding reak ke 8 2 operated continuously for the require interval. Lampl 90 at the calling end would not operate, since` the insertion of the plug 75 in the jack L, would open the locking circuitor relay 91, as explained. A better method for transmitting such a break signal is provided by the in the dist nt operator by causing a rators set with the L2' circuit of Fig. 7. The operating details lof this circuit will be explained below. At this point it is suicient to state thatthe circuit scribers restore their switches 33, lwhich relights the respective lamps 39 at the central offices, due to the flow of current through the lower windings of relays 53 only. The operators at both oiiices, upon noting the lighted lamps, disconnect the connecting cords from the loopand trunk jacks.

Built-up connection A built-up connection is necessary when there are no trunks extending directly between the central oi'ces to which two distant subscribers desiring communication' are connected. In such a case it is necessary to extend the connection through one or more central oices intermediate to those at which the calling and called lines terminate. A built-up connection is shown in Fig. 1 extending between stations 5 and 10.

Assuming station 5 to be the calling station, the procedure in establishing the connection as far as oiiice B is the same as that outlined above for a trunk circuit connection. The operator at oiiice upon receiving the signal due to the lighting of line lamp the R2 jack of an idle trunk 3 extending to the called subscribers oiii'ce C and then, after waiting for about one second, lugging the other end of the repeater cor into the R2 'ack of trunk 2, as shown bg' e reason why a pause o about one second is made before connecting the free end of'Y repeater cord 19 to trunk 2 is to permit a break signal of suicient duration to be transcord1`9 (Fig. 1)..

mitted over trunk 3 to light the line signal 90 associated with the trunk at voiiicezC. .If

desired, of course, both ends of the repeater f cord 19 may be plugged up, .s imultaneously and the usual break signal of one second duration transmitted. over-,trunk 3 to operate the line signal at oiiice C by operatlon o break ke 82 associated with the operators set at o ce B, which set, it will be recalled, is still connected tothe L, jack of trunk 2 thereat. The operator at oilice C, upon noting the` lighted line lamp 90, connects .her

operators set (Fig. 5) to the L2 jack of trunk 3, thus extinguishing the trunk line lamp. The operator at office C then communicates over the circuitwith the operator at oilice A, who still has her set connected tol a trunked circuit connection. Meantime the operator at oice B disconnects her set as soon as the operators at oflices A and Care in satisfactory communication. The operators at offices A and C remain on the line until the called subscriber answers and is in satisfactory communication with the calling subscriber, whereupon the operators at both offices disconnect.

When the call is complete, both subscribers restore the switches 33 associated with their printer sets, which stops the printer set motors and operates the loop signals 39 at offices A and C. The operators at these oflices, upon noting the signals, lthereupon remove the connecting cords 18 and 20, respectively, from the loop and trunk jacks. Since, however, the repeater cord 19 is still plugged up at office B, it will be necessary for the operator at office A to signalth operator at office B to tear down the connection. This disconnect si al is transmitted by inserting plug 136 o the circuit (Fig. 7) in the L2 jack of trunk 2 at ollce A and momentarily operating key 116. The operation of key 116 will cause the circuit of Fig. 7 to. transmit a long break signal of eight toten seconds duration over the trunk circuit, which will light line lamps 90 associated with trunks 2 and 3 at offices B and C in the manner explained above despite the fact that cord 19 is inserted in the R2y trunk jacks at office B.

The operation of the disconnect signal circuit of Fig. 7 is as'follows:

When plug 136 `isinserted in the L2 jack of the trunk, current normally flowing between batteries 108 and 109 in the trunk circuit now flows into the circuit of Fig. 7 over` the sleeve conductor, through relay 119 operating the same, through the off-normal contacts 128 of selector switch 120 and back to the trunk over the tip conductor. The operation of relay 119 connects battery to light line lamp 118, and prepares a circuit through vthe lower contacts of the relay to operate relay 121. When now, the operator momentarily operates the non-locking key 116, a circuit is completed from battery 122 through resistance 123, operated contacts of key 116, lower front contact and armature of relay 119 and through the winding of relay 121 to ground. Relay 121 thereupon operates and `curring immediately locks up to ground through its lower inner armature and Jfront contact. The

operation of relay 121 p repares an operating circuit for the operate magnet 124 of selector switch 120. This circuit is traced from battery 122 through the winding of magnet 124, lower outer front contact and armature of relay 121 to the right front contact of relay 131. The operatien of relay 121 also completes a circuit through its upper front contact and armature to ground, which causes the operation of relay 129. The operation of relay 129 completes a circuit from battery 134 through the right front contact and armature of rela 129, Winding of relay 130, to ground 135 through an interrupter 133 adapted to alternately open and close the circuit for approximate half-second intervals,

The function of the chain of relays 129 to 132, inclusive, is to transform the pulses from interrupter'133 which occur at approximately half-second intervals, into pulses ocat one-second intervals, which pulses are in turn utilized to step the selector switch 120 around one step for each such one-second pulse. Since the selector switch 120 contains only ten steps 127, this would mean that when the selector switch has been stepped completely around step 1 to step 10, about ten seconds will have elapsed. Interrupters of the type indicated at 133, thatA is, approximately a quarter-second on and a quarter-second off, are available in almost any central ofice, twhereas interrupters which break the circuit at one-second intervals are in general not available. Hence the necessity of the chains of relays 129 and 132.

Returning now to the circuit operation, the operation of relay 129, as explained, also .prepares a circuit for operating relay 131.

Thus when interrupter 133l completes the circuit totoperate relay 130' the first time, relay 131 will immediately operate on the Circuit from battery 134, left front contact and armature of relay 129, left back contact and armature of relay 132, armature and front contact of relay 130, right armature and back contact of relay 132 to ground through relay 131. The operation of relay 131 completes a circuit from battery 122 through the winding of magnet 124- to ground at relay 131 which causes the operation of magnet 124 and thus steps the brush arms 126 of the selector switch around one step. As soon as the selector switch is thus stepped olf its normal position, the off-normal contacts 128 are released,

which opens the circuit between the tip and Y sleeve of plug 136. This releases relay 119 to extinguish lamp 118, and also by interrupting the flow of current through relay 99' (Fig.

l6)A transmits a break signal over the trunk circuit. l

When now, relay 130 releases at the end of the first pulse of interrupter 133, it opens the operate circuit traced above-for relay 13,1, but a second circuit to maintain relay 131 operated is no w traced from ound throu h gl' g the' winding of relay 131, le front contact and armature thereof, winding of relay 132,

to battery 134 through operated contacts'of relay 129. Relay 132 operates over this circuit and prepares a circuit -to release relay 131 upon reception of the next pulse from the interrupter.v Thus, when the second. pulse from interrupter 133 reoperates relay 130, re-

y lay 131 releases due to the fact that a path for short-circuiting this relay is now completed through the right front contact and armature of relay 132, front contact and ar` mature of relay 130,1eft armature and front contact of relay 132 to ground. The alternate ground thus obtained maintains relay Y 132 operated. Relay -131 thereupon releases,

in turn releasing the operate magnet 124 of the selector switch. At the end ofthe second pulse yfrom interrupter 133, relay 130 releases, in turn yopening at its contacts the holding circuit for relay 132, which also releases, thus restori/ the chain of relays 130 to 132 to normal. n receipt of the next impulse of interrupter 133, relay 130 will again operate and initiateV a repetition of the cycle Y of operations traced above, which results in stepping the selector switch ahead one more step. It will therefore be seen that or each two impulses from the interrupter 133, the selector switch brush arms 126 will be advanced one step so that the selector switch is rotated one step for each approximately one-second interval.

Thel selector switch brush arms 126 will thus reach the tenth step in about ten seconds, in which position the winding of relay 121 will be .short-circuited causing the release of the relay, The short-circuiting path is of course traced from the lower front cony tact of relay 121 through step 10 and brush varms 126 of the selector switch to ground.

The release of relay 121 in turn opens the circuit to release relay 129 of the impulse circuit. The release of relay 129 in'turn dis- 122 through the winding of the release magnet, olf-normal contacts 128i to ground through the upper back contact and armature of relay 121. It will be noted that during the entire pe- 'riod that the selector switch is being stepped around, the circuit is opened between the tip a'nd sleeve conductors o f plug 136. Lamp 118 thus remains extinguished and a break signal is continuously transmitted over the --more than two subscribers stations.

trunk circuit to the Adistant end thereof. When the selector switch of Fi 7 is restoredv to normal, as explained, re ay 119 reoperatesand lights lamp 118. This tells thc operator at ofylce A that the break signal has been transmitted, whereupon she disconnects plug 136 from the L2 jack of the trunk. Upon termination of the break signal, relays `107 (Figi 6) at oiices B and C move to their respectivey marking contacts, which permits each condenser 93 to discharge through relay 91 thereat and operate the line lamps 90 at ofiices B and C as explained above. .On noting the lighted lamps) 90 associated with trunks 2 and 3, the operator at oiiice B inserts plug of her set' in the L2. jack of either trunk which extinguishes lamp 90 associatedwith the trunk in question in the mannerex-` plained above.` The operator at oflice B then challenges on the circuit by sending oice B or the like on her printer. Upon receivlng no response to this challenge., the operator disconnects cord 19 from trunks 2 and 3 and momentarily inserts a plug in the L2 jack of the other trunk circult to extinguish line lamp 90 associated therewith. The operator at office C upon noting lighted lamp 90 also connects plug 75 of her set to the L2 jack of trunk 3 thereat to extinguish the lamp and challenge in a similar manner. pon receiving no response, she disconnects.

Conference connection;

As was mentioned above, it is possible with the exchange system disclosed herein to establish connections which are common to Such connections are termed conference connections.7 A connection Vof this type is indi'- cated in Fig. 1 associating stations 5, 9 and 10.

Conference connections may be established which involve local stations, only, i. e., three or more stations having their loop circuits terminating at `a given central office. y*Conference connections may be established which involve local as well as distant stations. In such a case connections are extended over trunk circuits from the distant stations to the oice at which the local stations terminate and the plurality of stations are interconnected ina common communication circuit at such otlice. Finally, conference connections may be established involving ldistant stations only. For this case individual trunk circuit connections are extended from a plurality of stations to an intermediate oflice distant from all of the stations, and the various lines are interconnected at such intermediate oilice.

Conference connections of the first type, i. e. involving local stations only, may be established by means of repeater cords (Fig. 4) using the N jacks as well 'as the R1 jacks of the loop circuit (Fig. 2). That is,`.for a connection involving three local stations, one repeater cord would be connected between connected from the N jack of one of such interconnected loops and the R1 jack of the third loop. It will be obvious from thisexample that this procedure could be extended to include a larger number of stations.-

Connections of the second type which involve both local and distant stations may be established using a connecting cord to connect a local *station and a trunk circuit by means of the C1 and C2 jacks, respectively; the trunk circuit could then be connected to a second trunk circuit using a repeater cord between the R2 jack of the lirst trunk and the L2 jack of the second trunk; the second trunk could then be connected to a third trunk and lso on by means of repeater cords extending from the R2 jack of one trunk to the L2 -jack of the succeeding trunk in the chain; and linally, additional local stations could be added to that already co ected by means of repeater cords extending from the R1 jack of one local station to the N jack of the next local station, and so on. Y

Connections of the third type involving distant stations only could, of course, be established as indicated above by means of repeater cords each extending from the R2 jack of one trunk to the L2 jack of a second trunk. The insertion of repeater cords in the L2 vjack associated with certain of the trunks would, of course, disconnect the line lamp 90 from such trunks, but a disconnect signal could still be received on other trunks involved in the connection which had no plug inserted in the L2 jack thereof.

Operating details If, after a connection` has been established with a subscribers line (Fig. 2), the subscrib- .er desires to signal the operator, this may be accomplished by repeatedly operating and restoring the key switch 36. It will be recalled that after a connection is established, line currentflows through both windings of relay 53 at the central office and the relay is thus released. When,.however, key 36 is operated, it opens at the upper left contacts of `the key the S conductor of the loop and connects theT conductor of the loop to ground 62 through right and left operated contacts of key switch 36. This permits current to be typed up on the receiving unit 31, and thus a test can be made to determine if the printer is.in satisfactory operating condition.

It will be noted that the operation of switch 36 does not prevent the central office from operating the ringer 34 to signal the subscribers station. With switch 36 in its normal position, the ringer 34 obtains ground at 61, as explained above, throu h condenser 35. With switch 36 operated, t e ringer obtains ground at 62 through condenser 35 and the operated contacts of the switch, so that the operator at the central oliice, by applying ringing current to the S loop conductor may operate the ringer in any event.

Jack N, in addition to its use in establishing conference connections, as noted above, also provides means whereby printer for monitoring purposes may be associated with the loop circuit at the central office.

In case .of trouble to the circuit equipment to the left of line .fr-za the loop circuit may be transferred to spare circuit equipment by patching with `a connecting 'cord similar to Fig. 3 from jack M of the loop circuit in question to jack N of the spare circuit equipment,- at the same time inserting a wooden plug, or the like, in jack M of the spare circuit. With such an arrangement, a circuit would be traced from the sleeve conductor of jack' N associated with the spare circuit, over the sleeve conductor of connecting cord and through the sleeve conductor of jack M of the original circuit, and over the loop circuit 60 and back through the tip conductor of jack M of the original circuit, the tip conductor of the connecting cord to tip conductor of jack N of the spare circuit, through lower operated contacts of jack M of the spare circuit (due to the insertion of a'wooden plug therein) back `through the upper operated contacts ofjack N of the spare circuit to the tion that the spare equipment to the left of4 lines m-'would replace the similar equipment normally connected to the loop circuit.

The condenser 55 in series with a suitable resistance is connected between the left armature and back contact of relay 52 to prevent possible buzzing of relays 52 and 53 when the subscriber operates switch 33 to initiate a call. When switch 33 is operated, current flows from battery 50 through upper winding of relay 53, over the loop circuit and back through non-operated contacts of relay 54'to ground through left armature and back contact of relay52, as explained above. lThis current operates relay 53 which, in turn, completes a circuit to operate relay 52, as explained.

The operation of relay 52 maintains relay -53 operated [thereafter by replacing the 'which might possibly-cause relay 53 to fall olii' and thus release relay 52 before the operation of relay 52 is complete. This would cause the cycle of operations described to start allover again, with the result that relays 52 and 53 would buzz continuously thereafter.

` To prevent this possible effect, the condenser 55, in series with af suitable resistance, is connected as shown. With relay 52 released,

, no charge would accumulate on condenser 55.

When, now, switch 33 is oper'ated, relay 53 would be' energized through its upper winding, as explained, and cause the operationof relay 52. In this instance, however, as the left armature of relay 52 movesolf ofits back ,4 contact, current from battery A continues to iow through theupper winding of relay. 53 to charge up condenser 55. This ilow of current maintains relay 53 operated until the left armature of Irelay52 is operated against its front contact, whereupon the positive battery 51 maintains relay 53 operated therei after. The resistance in series with condenser 55 merely serves to limit this charging Jcurrent and thus minimizes arcing at the left armature and back contact of relay 52.

The variable resistance 56 is connected inseries in the Sl conductor of the loop circuit inorder to adjust the loop resistance to a constant valuel for all loops. The resistance 72 is inserted in the 'I'` conductor, as shown, in order to prevent excessive Vcurrents from ilowin over this conductor to ground at 61 after e subscriber has switched olf by restoring switch 33 but before the operator has disconnected her operators set repeater cord or trunk circuit from the L1, R1 or C1 jack of v the loop circuit, since, for any of these conditions, batte is connected to the T loop conductor at t e central oiiice.

With reference to Fi 6, resistance 96 associated with the C2 jac is connected in the current supply circuit when the trunk is dis-- connected and also when an operators set 1s connected to the L2 jack or a repeater cord to the R1 jack. Under such circumstances,

resistance 96 which is equivalent to the resistance of a subscribers loop maintains the direct signaling current at its roper value. When a loop is connected to t e trunk circuit, the insertion of a connecting cordplug in the C2 jack replaces resistance 96 by the loop circuit.

It is necessary to use the repeater cord for connecting one trunk circuit to anothervin order to properly close through the D. C.

rconnecting cord, negative battery 109 in one trunk would oppose negative batter 109 in the other trunk so that s acing signa s would normally be transmitte to the distant ends of both trunks. v

The operators printer set indicated schematically in Fig. 5 is of the recently developed, mechanically selected type in which the selection of characters during the actual printing operation is accomplished by mechanical means controlled through the signal magnet 78 operating in accordance with the received signal ulse rather than by means of a group of se ecting relays, as is the case of certain well-known types of rinters.

The circuit is so arranged that with magnet 78 operated, it prevents rotation of the receiving cams associated with the printer mechanism. During reception of signals, the magnet 78, as will be explained, releases in accordance with the received signaling pulses, and the release thereof, according to the initial or start pulse for each character transmitted, permits rotation of the receiving cams once. If, during abnormal conditions, the magnet 78 is released continuously, it will permit continuous rotation of the receiving cams. t is undesirable, of course, owing to wear and tear on the apparatus, to have thefreceiving cams rotating continuously while the printer set is not in use.- For this reason, it is necessary to provide means whereby the magnet 78 is operated continuously while the operators circuit is disconnected `:lrom the line, as well asto have it operated continuously while connected to a line and no si als are being received. The slow release re ay 84:v controls a vcircuit for maintaining themagnet 78 operated while plug 75 is disconnected from a subscribers line or a trunk circuit, as will now be explained. y

When the plug 75 is inserted in the jack of a subscribers line or of a trunk circuit, the current supplied therefrom, in flowing through the line winding of relay 77, will operate the same against its marking contact.

The operation of thepolar line relay 77 against its marking contact will complete A a circuit to operate magnet 78 traced rom batte 83 throu h resistance 89, armature and contact o relay 77, winding'of slow release rela 84 and through the winding of magnet 8 to ground at 88. Magnet 78 thus remains operated while no singals are being received and prevents rotation of the receiving cams associated therewith. The slow release relay 84 also operates-over the circuit traced aboveand opens a second operating circuit for ma et 78 traced from battery 83, throughreslstance 89,-back conv through magnet 78 to ground at 88.

Durmg reception of signals, the pola-r line relay 77 is operated against its spacing contact for cach interruption in the line current, in turn, opening at its contacts the operating circuit traced above for magnet 78. .Magnet 78 thus follows'the yoperations of polar line relay 77 and causes the messages to be typed up in accordance with the signals received. Relay 84, being slow to release, does not follow the rapid current interruptions through its winding but remains operated continuously while signals are being received and thus prevents closure of the second operating circuit for magnet 78 which was traced above. As the result of this feature, magnet 78 is under absolute control of polar line relay 77 while the operators set is connected toa subscribers line or trunk circuit.

Upon disconnection of the operators set from the line, however, the polar relay 77 is operated continuously 'against its spacing contactvdue to current supplied to its auxiliary Winding from battery 88 and through resistance 87 to ground. This interrupts the circuit traced above from battery 83 through the winding of relay 84 and magnet 78 to ground, with the result that magnet 78 releases, immediately to be followed, after a short interval, by the release of relay 84. Upon the release of relay 84, the second operating circuit for magnet 78 traced above through resistance 89, backlcontacts of relay 84 and through resistance 85 to ground through the winding of magnet 7 8v which causes the reoperation of the magnet and thus prevents rotation of the receiving cams. Relay 84 remains thus released during the entire interval that the operators set is disconnected from a line with the result that the receiving cams are prevented from rotating during such interval. Since, upon disconnection of the operators set .from the line circuit, magnet 78 releases momentarily during the interval between such disconnection and the falling oli of relay 84, the receiving cams will continue to rotate during such interval. No difficulty results from this fact, however, beyond the possible printing of a single false character.

The circuit comprising the resistance 86 in series with a capacity connected between the marking contact and the armature ,of polar relay 77 is merely a spark killer for miniv mizing arcing at the contacts of relay 77 during operation.

The plug and jack arrangement 81 provides a flexible system in case of trouble to the operators printer set, in which case a substitute printer set may be associated with the plug 75 by merely plugging the same into the jack.

What is claimed is:

l. A telegraph exchange system comprising in combination, a pluralit of central ofices, loop circuits extending rom each oiiice to a pluralit-y of subscribers telegraph stations individual to such oilice, telegraph trunk circuits interconnecting said oces,

link circuits at each oliice for interconnecting the loop circuits terminatingl thereat, or for connecting a loop circuit to a trunk circuit for communication purposes operators telegraph sets at the central oflices with means independent of said link circuits for opera.- tivel associating the same with a loop or trun circuit.

2. A telegraph exchange system comprising in combination, a pluralit of central offices, loop circuits extending rom each of'ce to a plurality of subscribers telegraph stations individual to such oiice, telegraph trunk circuits interconnecting said oliices, a plurality of two-conductor link circuits at each office for interconnecting the loop circuits terminating thereat or for connecting a loop circuit to a trunk circuit for communication purposes, operators telegraph sets at `the central offices with means independent of said link circuits for operatively associating Athe same with a loop or trunk circuit, a single line signal individual to each loop circuit aty the corresponding oice whereby a subscribers station may at any time signal the central office, means whereby a central office may signal any subscribers station connected thereto, and means whereby an operator at one cenJ tral oiiice may si al over a trunk circuit an operator at the dlstant oice.

3. A telegraph exchange system comprising in combination, a plurality of central ofices with loop circuits extending from each oiice to a plurality of subscribers telegraph stations individual to such oliice, telegraph trunk circuits interconnecting said oiiices, link circuits at each oiice containing two-way telegraph repeater elements for interconnect- 'ing the loop circuits thereat, other link circuits at each oiiice for directly connecting a loop circuit to a trunk circuit for communication purposes, means for signaling the central oiice from a subscribers station, means for signaling any subscribers station from the corresponding central oitice, and means for signaling a distant oiiice over a trunk circuit from a given oiice.

4. A telegraph exchange system comprising in combination, a plurality of central of-. ices with loop circuits extending from each such oiice to a plurality of subscribers telegraph stations individual to such oiiice, telegraph trunk circuitsinterconnecting said ofces, link circuitsat' each oliice containing two-way telegraph repeater elements for interconnecting the loop circuits thereat, other link circuits at each ot'tice for directly connecting a loop circuit to a" trunk circuit for .10 tions, said means .being communication purposes, means for signaling the central oice l from a` subscribers station, means for signaling any subscribers station from thecorresponding central oftice, and means for signalinga distant oice over a trunk circuit from a given oice, means associated with the loop circuits at each oiiice adaptedl to supply telegraph signaling current. to the correspondin subscribers starther adapted to supply similar current'to any saidtwo-way repeater element when connected between loops, means whereby a trunk circuit supplies such signaling current to aloop circuit upon lconnecting the same therewith, and operators telegraph sets at each oice with means for associatin the same with 'a loop or trunk circuit where y such circuit supplies signaling current to said'operators set. 2 5.v A telegraph exchange system comprisy ing in combination,'a central ollice having a pluralit of subscribers stations each equippcd wit a printing telegraph set individual y connected thereto over loop circuits, tele- 25. graph trunk circuits extending from said oilice'to other similar yoices, link circuits at said oice for interconnectingsaid loop circuits or for connecting trunk and loop circuits for communication purposes, o erators pr-iit telegraph sets at said o ce with nieans or operatively associating the same 4 Y.with loop or trunk circuits, aline signal associated with each loop at the central station,

switching means at each subscribers station" operable to light the corresponding line signal and' to operatively associate the printer set at said stationwith the loop circuit, means mresponsive to the association of an operators set with said loo for extinguishing said line si al, addition switching means at said su scribers station foriiashing said line signal thereafter, means responsive to the restoration` oi said switching means for reoperating said signal, and means responsive to the disas'sociation of said operators set from said loop for extinguishing the same.

6. a telegraph exchange system as set forth in claim 5, slow acting m/eans associated with each operators printegset adapted to render such set-inoperable whileudisassociated from a loop or trink.-J circuit but adapted to giace such set under control of the telegraph e current when so associated. 72A telegraph exchange system comprisu igv in combination, a central oice with loop circuits extending therefrom to a'plurality of subscribers stations each-.- equipped with a printing teleg-ra h set, tele ph trunk circuits extendin om said o ce to other Asimilar oQces, circuits at said o'iceA forinterconnecting said loops orloops and trunks for communication p es, operators printer sets `at said oce with means for operatively 'associating the 'saine with any W said loop or trunk, signalingmeans asso- [trunk over rsai oioad with soia loops at said stations responsive-.to ringing current, means at said oice vto apply such current to any said loop to call line signal, additional means at said station` for flashing said line signal thereafter, and means re onsive to the subsequent restoration of said station switching means before said link or operators set is disconnected forv relighting said line signal to obtain'a discon-A nect, means responsive to suchidisconnection for extinguishing said si al, and means whereby an 'operator at sai oice' may signal a distant operator over a trunk circuit.

8. A telegraph exchange `system compris ing in combination, a central oilipce with loop circuits individually connecting the same vto a pluralityof subscribers stationsftelegraph Y trunk circuits extendin other. similar oces, -lin lcircuits containing two-way telegraph, repeater elements for inyterconnecting said loop circuits at said office, other link circuits thereat' for directly connecting a loop to a trunk circuit, a printing telegraph setat each said station responsive tolswitching means thereat for operatively associating said rinter set with said loop, means at said o ce responsive to such association for supplying line current over said loop to said printer9 set, operators printer sets at saidy oilice with means for associati'ng'they same with a loop. or trunk, means responsive from said otlice to to such association witha loop or trunk circuit for supplyingy line current from said loop or trunk to the operators set thus oon-r nected, and means responsive to the connection of a trunk to a loop circuit as aforesaid adapted tol 'jsup ly line current from 'said loopA to'i said subscriberis printer set, means whereby any said station may signal said oice and vice versa, and means whereby an o rator at said oice may signal a distant operator over a trunk circuit.

system compris- 9. 4A.v telegraph exchan ing a plurality of centr oices, pluralities of subscribers? telegraph stations individual.

to said oices and individuallyconnected thereto over loop rcircuits, telegraph trunk circuits interconnecting said otlices,1ink circuits at each olicel f or interconnecting the' loop circuits thereat with each link circuit containing a two-way telegraph repeater element, meansresponsive to such interconnection of a'link circuit adapted to supply line current to .the 'loop`circuits and repeater element thus associated, other link circuits at eacliollice. fordreotly oonnectingatrunkcircuit to a loop circuit thereat whereby line current for the loop circuit is supplied from telegraph means associated with the trunk circuit, operators telegraph sets at each ofice,

. scribers station may signal the corresponding central oiice, means whereby an operator may signal a subscribers station, and means individual to said trunks whereby an operator at one oiiice may signala distant olice, said means responding to signaling current of one length when said trunk is idle at the distant oce, and responding to signaling current of a dierent length when said trunk is associated with a link circuit thereat.

10. In a telegraph exchange system in combination, a plurality of central oiices, loop circuits extending from each olice to a plurality of subscribers telegraph stations individual t0 said olice, telegraph trunk circuits interconnecting said oiices, link circuits thereat for interconnecting .said loops or said loops and trunk, operators telegraph sets at each oilice with means independent of said link circuits for operatively associating the same with a loop or trunk circuit, means independent of said link circuits for supplying telegraph line current to said loops, trunksand operators sets, and signaling means inde-A pendent of said link circuits whereby a subscriber may signal the central oiice and vice Versa, and whereby any operator at one ofice may signal over a trunk circuit an operator at a distant oice.

11. In a telegraph exchange system a plurality of subscribers stations each equipped with telegraph sending and receiving means, loop circuits individually connecting said stat-ions to a central olice with each loop terminating in a plurality of jacks thereat, link circuits at the central oice for interconnecting said loops, each link comprising a twoway telegraph repeater connected between plugs, an operator s telegraph set at said offce terminating in a plug, means individual to said loops and adapted, upon association of a link circuit with a first loop jack of said lurality for supplying linekzurrent to the oops and 0repeater element thus connected, said means being further responsive to the association of said operators set with a second loop ack of said plurality for supplying line current to the loop and operators set, telegraph trunk circults terminating in jacks at said olice, additional link circuits thereat for directly associating a trunk circuit with a third loop jack whereby line current for the loop is supplied from telegraph means individual to said trunk.

12. In combination a signal 'transmission circuit extending between two points, means at each point for transmitting signaling current over said circuit to the distant point, signaling means at each point responsive to said signaling current received from the distant point, said signaling means comprising at each point a line relay, means responsive to the operation of said line relay in one sense by said signaling current for charging a condenser from a direct potential source through a high resistance, whereby a predetermined time interval is required to charge said condenser fully, means responsive to the operation thereafter of said line relay in the opposite sense, to discharge said condenser through a second relay to operate the same provided said condenser is charged approximately to the full potential of said source, a

locking circuit for said second relay completed upon operation thereof, a line signal responsive to the operation of said second relay, means at the point in question for extinguishing said line signal.

13. In a telegraph exchange system in combination, a telegraph trunk circuit extending between two central oiiices, signaling means associated therewith whereby an operator at either oiice may signal the distant ofce, said means comprisingat each olice means for transmitting a break signal over said trunk, a normally operated line relay responsive to such a break signal transmitted over said circuit from the distant office, a circuit completed upon release of said line relay during reception of said break signal for charging a condenser from a direct potential source through a high resistance whereby a certain time interval is required for charging said condenser, means responsive to the reoperation of said line relay upon terminationof said break signal for discharging said ycondenser through a second relay to operate the' same provided said condenser had accumulated approximately a fullcharge, a locking circuit for said second relay completed upon operation thereof, a line signal responsive to the operation of said secondrelay, means at the called oice for extinguishing said line signal.

14. In a'- telegraph exchange system comprising a plurality of central offices, pluralities of subscribers telegraph stations individual to said oflicesand individually con nected thereto over loop circuits, telegraph trunk circuits interconnecting said o'ices, operators telegraph sets at each oiiice, and

link circuits thereat for interconnecting saidloops and said loops and trunks, means individual to said trunks whereby an operator at one olice may signal the distant oce, said means comprising for each trunk at each oice a normally operated line relay releasing upon reception f a telegraph break signal transmitted from the distant oilice, means responsive to the release thereof for charging a condenser from al direct potential source through a high resistance whereby a certain las time interval is required for charging said condenser fully, means responsive to the reoperation of said line relay upon termina-Xp tion of said break signal for discharging said condenser to operate a second relay provided said condenserV has accumulated a proximatel'y full charge, a locking circuit or said se'cond relay completed upon operation thereof, a line signal responsive to the operation of said second relay, means for associating any said operators set with said trunk circuit at the calling oflice for transmitting said break signal to the distant ofce to operate said line signal thereat, means whereby such association prevents operation of the'l signaling means associated with said trunk at the calling office, means whereby the connection of an operators set to said trunk at the called oice extinguishes said line signal thereat,

'so means whereby the association of a link circuit with said trunk at the called office increases said condenser charging resistance whereby an appreciably longer break signal than that first mentionedis required toreoperate'said linesignal with said link circuit thus connected.

15. In a telegraph exchange system as set.-

:forth in claim 14, a device or transmitting said appreciably longer break signal to reoperate said trunk line with a link circuit connected to said trunk, said device comprising a pair of normally closed'contacts with means for serially connecting the same in said trunk circuit at the calling oilice, automatic 35 tuning and switching means responsive to manually operable start means for thereafter opening said serially connected contacts a predetermined time interval ffor transmitting a break si al to reoperate said distant line signal, an means associated with said device to indicate the reclosure of said contacts upon termination of said break signal.

16. In combination, a` printin telegraph receiving set, a line-relay adapte to be energized in accordance wit telegraph line current, slow acting means responding to the magnetic means adapted while energized continuously to prevent rotation of said cams, but permitting such rotation while deenergized continuously or intermittently 'in accordance with telegra h signaling currents, means responsive to t e association of said rinter set with a normally closed telegraph line for energizing said electromagnetic.I

means in accordance with said line current, but responding to a sustained interruption -f said line current orto the disassociation of said printer from such a line, to energize said electromagnetic means continuously from a local current source 4for preventin rotation of said cams during such a' perio 18. In combination with a printing telegraph set including driving motors and receiving cams wherein during operation of the driving motors thereof, the rotation of the receiving cams is controlled b electromagnetic -means adapted while energized continuously to prevent rotationof said cams, but permitting such rotation while deenergized continuous y or intermittentlyin accordance with telegraph .signaling currents, means responsive to the association of said aprinter set with a normally closed telegraph line for energizing said electromagnetic means in accordance with said line current, but responding to a sustained interruption of said line current or to the disassociation of said printer from such a line, to'energize said electromagnetic means continuously-from a local current source for preventing rotation of said cams during such a period, said means comprising a line relay responsive to said line current, a slow release relay responsive to said line relay and operable continuously Ythereby during the] continuous or intermittent energization of `said line relay in accordancewith telegraph signaling currents, means responsive to the ing said electroma sive control of sai line relay, means responsive to the release ofsaid slow release relay during a sustained release of said line relay consequent upon a sustained interruption to said line current or the disassociation of said printer from said line, for energizing said electromagnetic means from a se arate cur-,- rent source to prevent rotation o said cams.

du'rin continuance of lsuch condition.

19. n combination with a printin telegraph set including receiving cams w erein during operation thereof, the rotation of the- `operation of said slow release relay for placetic means under excluc receiving cams is controlled lby electromagnetic means, means responsive to the association of said printer set withv a normally closed telegraph line for energizing said means in accordance with said line current for rotatin said cams in accordance with telegraph signaling currents, and means responsive to the ldisaociation of said printer set from said line for energizing said means from a local current source -whereby the rotation of said cams is prevented during such disassociation. A

20. In combination. with a printin telegraph set including receiving cams werein 

